1) Field of the Invention
The present invention relates to an organic light-emitting device, an organic light-emitting display apparatus including the organic light-emitting device and a contact wiring structure that electrically connects its emission control circuit to the organic light-emitting device, and a method of manufacturing the organic light-emitting display apparatus.
2) Description of the Related Art
Organic light-emitting displays are attracting attention as candidates for flat display devices instead of liquid crystal displays. The organic light-emitting displays differ from the liquid crystal displays in that organic light-emitting diodes (LEDs) generating light are employed. In other words, the organic light-emitting displays do not require backlighting which the liquid crystal displays need. An organic LED has high speed response, high contrast, and high visibility. Further, an organic light-emitting display using the organic LED has a relatively simple structure, which is advantageous in view of manufacturing cost.
The organic LED has a mechanism that emits light by charge injection. Potential at each of the organic LEDs, particularly in a large-size display, tends to vary depending on its position. Of organic light-emitting displays referred to as top emission structure, one having anode line structure is considered to be promising at present, in which the anode electrode of the organic LED is extended up to a current source. This is because the anode electrode has generally low resistance and thus occurrence of a voltage drop between the current source and the organic LED is suppressed.
FIG. 9 is a sectional view illustrating a part of the structure in a conventional organic light-emitting display having the anode line structure. This organic light-emitting display has a multilayer structure in which a planarizing layer is formed to cover an emission control circuit formed on a substrate, and the organic LED is formed on the planarizing layer. As shown in FIG. 9, the organic LED 104 including an emitting layer 109 is located on the planarizing layer 106, while thin film transistors 102 and 103 constituting the emission control circuit are located on the substrate 101.
This organic light-emitting display also has a contact wiring structure in which the planarizing layer 106 has a hole for exposing a part of a conductive layer 105 at the bottom of the hole and the organic LED 104 is electrically connected to the thin film transistor 103 via the conductive layer 105. The cathode layer 110 is extended up to the exposed part of the conductive layer 105, to be connected to the conductive layer 105 electrically. Such a contact wiring structure is disclosed in, for example, “A 13.0-inch AM-OLED display with top emitting structure and adaptive current mode programmed pixel circuit”, T. Sasaoka et al, SID Tech. Dig., 2001, pp. 384 to 387.
Since the light emitted from the organic LED 104 is output perpendicularly upwards, the cathode layer 110 located on the organic LED 104 needs to have excellent optical transmittances. Therefore, to ensure excellent optical transmittances, the cathode layer 110 is formed very thin, and for example, it has a film thickness of not larger than 10 nanometers. On the other hand, the planarizing layer 106 needs to have a larger film thickness to reduce parasitic capacitance, and has generally a film thickness of about from 2 to 5 micrometers. The cathode layer 110 extended up to the hole is disconnected at the side of the hole because of its thinness, in some cases. Therefore, to prevent disconnection, a connection layer 108 having a sufficient thickness is located below the cathode layer 110 and over the sides of the hole.
Since the connection layer 108 can be formed by the same process as the anode layer 107, a new process is not required for preparing the connection layer 108. As a result, disconnection between the organic LED 104 and the emission control circuit can be prevented, without increasing the manufacturing cost.
There are two problems with the organic light-emitting display having the structure shown in FIG. 9. The first problem is deterioration of the electric conductivity on the contact area between the connection layer 108 and the cathode layer 110. The connection layer 108, reinforcing the cathode layer 110 near the hole of the planarizing layer 106, is formed simultaneously with the formation of the anode layer 107, after depositing the planarizing layer 106 on the substrate 101. Therefore, the connection layer 108 is formed of the same material as the anode layer 107, and shows the same chemical characteristics as the anode layer 107. The anode layer 107 has not only a function as an electrode of the organic LED 104, but also a function as a hole injection layer. Therefore, the anode layer 107 is formed of a metal material having a large work function such as platinum (Pt), so that holes can be effectively injected to the emitting layer 109. Since the material of the connection layer 108 is the same as that of the anode layer 107, the connection layer 108 has a large work function.
On the other hand, the cathode layer 110 has not only a function as a cathode electrode of the organic LED 109, but also a function as an electron injection layer. The cathode layer 110 is formed of a material having a small work function, so that electrons can be effectively injected to the emitting layer 109. As a result, the work functions of the connection layer 108 and the cathode layer 110 differ largely from each other, and hence metal materials having largely different work functions come in contact with each other. As a result, the cathode layer 110 is effectively oxidized and corroded by a small amount of oxygen or moisture in air, because of its small work function. Particularly, the oxidation-reduction is promoted by current flowing into the cathode layer 110 and the connection layer 108. This leads to an increase in a resistance or a disconnection in the wiring structure.
Since the oxidation-reduction occurs in the contact area between the cathode layer 110 and the connection layer 108, an electrical connection on the contact area is broken, to cause disconnection between the organic LED 104 and the thin film transistor 103. As a result, the organic LED 104 cannot perform the function as a light-emitting device.
The second problem relates to a general organic LED. The cathode electrode and the anode electrode adjacent to the organic LED need to be formed of a material having a predetermined work function to perform charge injection effectively. Therefore, the option for the material of the cathode electrode and the anode electrode becomes very narrow, and it may be necessary to use a material, which is not always excellent in properties other than work function. For example, metal materials having a small work function and used for the cathode electrode, for example, magnesium (Mg) or calcium (Ca), easily react to moisture or oxygen in air. When such a metal material is used, oxidation is likely to occur due to the moisture and oxygen of about 1 parts per million, and hence it is necessary to cover the cathode electrode from air completely.
Further, the metal materials having a large work function and used for the anode electrode, for example, platinum (Pt) or iridium (Ir), tend to be more expensive than the metal materials such as Al used for ordinary conductive layers. These metal materials have not always excellent adherence to the planarizing layer 106 formed of a polymer, and there is the possibility that the anode layer 107 peels off from the planarizing layer 106 due to aged deterioration.